Replace content translation with entity translation (and add an upgrade path)

Once #49 gets in, what stopping us from using entity translation instead of content translation

Pros: unified way of handling translation (nodes, taxonomy, ...) no need of i18n

Cons: still hard to manage translations of menu, block and other things

Related - https://github.com/backdrop/backdrop-issues/issues/4198 Content translation improvements

As people weigh in here with reasons they prefer (or were forced) to use entity translation, I would love if it we could add those reasons to that issue too -- since many of those improvements will be able to be included before 2025 :)